The performance of long-haul fiber-optic communication systems depends largely on the optical signal-to-noise ratio (OSNR). The OSNR is a conventional measurable characteristic of the quality of a signal carried by an optical telecommunication link. The dominant noise component in an optical communication link is typically unpolarized amplified spontaneous emission (ASE) noise. ASE noise is a spectrally broadband noise source contributed by the optical amplifiers in the link. In practice, the OSNR must therefore be measured somewhere along the link, for example at the receiver end. Current state-of-the-art systems achieve high spectral efficiency using both sophisticated modulation schemes and polarization multiplexing (PM). However, traditional OSNR measurement techniques fail in cases where both densely-packed channels and PM signals are combined. That is, the ASE noise spectrum generally cannot be measured outside of the signal-spectrum bandwidth because the channels are too closely spaced. Meanwhile, in-band measurement methods of the polarization-nulling type, which rely on the fact that the signal is 100% polarized whereas ASE is unpolarized, also suffer from drawbacks because the overall PM signal is also unpolarized.
A method of measuring noise level on PM signals using an acquired optical spectrum trace of the signal was proposed in co-owned U.S. Pat. No. 9,112,604 B2, the disclosure of which is incorporated herein by reference in its entirety. This method is based on the knowledge of the spectral shape of the data-carrying signal contribution provided by a reference signal. From this knowledge, the data-carrying signal contribution and the ASE-noise contribution, which otherwise appear as merged together on the optical spectrum trace, may be mathematically discriminated from each other. Knowledge of the spectral shape of the signal contribution may be derived from a prior acquisition of a reference signal taken. For example, the reference signal can be acquired at a different point, generally upstream, along the same optical communication link, where the OSNR is known or where the signal can be considered free of ASE noise. As such, the reference signal originates from the same optical transmitter as the signal under test. The method described in U.S. Pat. No. 9,112,604 B2 assumes that, within the optical signal bandwidth, the spectral shape of the signal does not significantly change along the communication link. The signal contribution of such a reference signal is therefore considered to be spectrally representative of the signal contribution of the signal under test. U.S. Pat. Appl. Pub. Nos. 2014/0328586 A1 and 2016/0127074 A1, the disclosures of which are incorporated herein by reference in their entirety, include provisions to account for spectral shape variations originating, for example, from nonlinear effects.
A method for determining in-band OSNR and other quality parameters in optical information signals, for example PM signals, is disclosed in U.S. Pat. Appl. Pub. No. 2016/0164599 A1. The method involves measuring an optical power spectrum of a noisy signal, for example by means of a conventional optical spectrum analyzer, and subsequently measuring correlations between a predetermined pair of spaced-apart time-varying frequency components in the optical amplitude or power/intensity spectrum of the signal by means of two optically narrow-band amplitude or power detectors. The in-band noise in the signal may be determined from the correlation measurement. A measurement of the signal power may be used to determine the OSNR based on the determined in-band noise. A drawback of this method is the complexity and high cost of the required apparatus, notably involving two full coherent receivers.
Challenges therefore remain in the development of techniques for discriminating signal from noise in optical signals used in telecommunication applications.